Currently, mods are indeed
possible on switch, but stick with modding the switch version personally, to yourself. Uploading switch mods is highly risky, and just not worth it. I'm currently the only person I know who has a modded switch with Cave Story+ mods.
Notes on switch modding as of current:
-TSC is un-encrypted, pretty much text files. You can edit them easily with a program like Notepad++ (Booster's Lab / Cave Editor will flip their shit if you try opening the TSC files for the switch version)
-The menus get really wonky when you mod. I have no idea how to fix this, but a lot of text is just replaced with
StringTable::InvalidKey
-Maps must
always be widescreen in order to look right. This room looks odd because I ported a 4:3 room for a mod I made a long time ago.
-Music modding is possible if you know how to replace oggs
-Graphic changing mods are possible if you know how to edit the pngs (and know what each file is)
Thanks, hope you found this info helpful!